NMR studies of exchange between triazine rotamers

Proton and C-13 solution-state spectra were obtained for a substituted tria zine and assignments made for the signals using different solvents. Site-ex change effects were observed in H-1 spectra at different temperatures below ambient for a CD3OD solution. The bandshapes were fitted for three-site ex change and the relevant rate constants extracted. The exchange was attribut ed to internal rotation of the substituents about the triazine ring. Activa tion parameters were calculated: for exchange between one pair of sites Del ta H-double dagger = 71 +/- 4 kJ mol(-1) and Delta S-double dagger = 33 +/- 15 J mol(-1) K-1, and for a second pair Delta H-double dagger = 72 +/- 5 k J mol(-1) and Delta S-double dagger = 39 +/- 26J mol(-1) K-1, whereas direc t exchange between the remaining pair of sites is negligible. Exchange invo lving a fourth rotamer affects the spectra at lower temperatures, which ass ists in a partial assignment of the observed peaks to the rotamers. These r esults are discussed in relation to molecular modelling information and lit erature values. Solution-state and solid-state C-13 spectra were also recor ded but the latter are broad, making detailed assignment difficult. Copyrig ht (C) 2000 John Wiley & Sons, Ltd.